The present invention relates to refrigeration systems which incorporate a plurality of electrically actuated solenoid fluid flow control valves. More particularly, the present invention relates to a refrigeration control system which incorporates a microprocessor for monitoring and controlling the system's operation by, for example, obtaining signals indicative of the flow rates through the solenoid valves.
A valve which meters fluid flow therethrough in accordance with flow demand, i.e., how much volume of fluid is permitted passage through the valve for a given period of time, typically operates in connection with a control signal developed by sensing a system condition. If the value of the sensed condition is different than a predetermined desired operating point, a control signal is produced for changing the fluid flow opening of the valve to meet the changed flow demand.
A fluid flow control valve is generally designed to operate over a range of flow demands. Typically for such fluid flow valves, the response curve defining the relationship between the sensed condition and the resulting fluid flow rate through the valve is linear over this operating range.
For such a prior-art valve, a given change in the sensed condition at a low demand flow rate will produce a certain change in the flow rate through the valve. This change in flow rate relative to the operating demand flow rate can be expressed as a percentage change. When the valve is operating at a high demand flow rate, the same given change in the sensed condition still produces the same amount change in the flow rate. This amount of change, when expressed as a percentage of the flow rate at the higher operating demand condition, will be less than it was for the lower operating condition. Thus, to effect the same percentage change in the flow rate at the higher demand level, a greater change in the sensed condition must occur. This greater change in the sensed condition to effect the proper change in flow rate represents a disadvantage in these prior-art valves. A control system is more stable when the system can be controlled to the desired operating point responsive to small changes in the sensed condition.
An additional disadvantage in these prior-art valves is that the set points or operating points for the sensed condition change depending upon what the demand flow rate through the valve happens to be. Thus, for a 30% demand condition, the operating point would be one value while a 60% demand condition would require a second higher operating set point.
A further disadvantage present in these prior-art flow control valves is characterized by a hysteresis error between the control signal applied to affect a flow condition and the actual flow condition which results. In an error free system, a given control signal should produce a particular flow rate through the valve. Where hysteresis errors are present, changing the control signal a given amount to effect a given change in the flow rate as predicted by the system control transfer function does not necessarily result in such desired change.
This hysteresis error is due to the valve's inability to achieve the desired orifice opening because of mechanical errors, magnetic errors, etc., in the valve's components. In a closed loop control system, such hysteresis errors will result in a continual "hunting" effect by the control signal since any demand must exceed the hysteresis error before any actual change in the flow rate is affected, i.e., the system is essentially underdamped. Such control never actually catches up to the demand. This hysteresis effect is the same whether the demand flow rate increases or decreases.
It is also known in the refrigeration art to provide subcooling of the liquid refrigerant in a low temperature refrigeration system to reduce the energy required for the low temperature system to meet its cooling demand. One of the disadvantages of the prior art methods to accomplish this function was that often times a reduction in the system compressor capacity occurs as a result of a decrease in the cooling load on the low temperature refrigeration system. In the prior art, the amount of subcooling does not change, even though it is no longer needed at that previous level of subcooling. As a result, the maximum energy reduction possible from the subcooling arrangement is not realized because the same amount of energy to provide the subcooling is continued.
Yet another problem in prior art refrigeration systems is the logging of compressor lubrication oil in the evaporator coils of the system. While, ideally, the compressor lubrication oil should remain in the compressor(s) where it is needed, a portion always mixes with the liquid refrigerant and is circulated through the system. From the condenser coil to the evaporator coils, the refrigerant is in its liquid state and transportation of the oil is quite easy. Coming out of the evaporator coils, the refrigerant is in its gaseous state and transportation of the oil is not as easy. In fact, quite a lot of the oil begins to collect in the evaporator coil(s). This oil in the evaporator coil somewhat reduces the efficiency of the evaporator, but more importantly, the accumulated oil is no longer available to the compressor(s) where it is needed.
The prior art has attempted to solve the problem of oil in the evaporator coils by controlling the geometry and line sizes of the piping of each evaporator coil from its output end to a common manifold at the suction side of the compressor(s). Each evaporator coil therefore, had to have its outlet end piped from the cases to the compressor room so that control of velocity of the gaseous refrigerant could be controlled. Wide velocity differences would produce less effective return of the oil. A single return suction line from the cases, i.e., the evaporator coils, to the compressor(s) was not possible where multiple evaporator coils are used, a situation which is true in most refrigeration systems.
Accordingly, it would be advantageous to provide a solenoid flow control valve which operates with essentially zero hysteresis error thereby to achieve the accurate control of the flow rate therethrough. It would also be advantageous to provide a solenoid flow control valve which could be operated in a closed loop control system with only one set point regardless of the flow demand condition through the valve, with a control response function which produces the same percentage change in flow rate to a given sensed condition change at a high demand flow as occurs for the same condition change at a lower demand condition.
It would also be advantageous to provide a microprocessor controller for effecting system wide monitoring and control of a plurality of such valves, and to provide feedback control of one such valve in a first refrigeration system providing subcooling to a second refrigeration system when there has been a change in the second system compressor capacity indicative of a reduced cooling load.
It would also be advantageous to provide a method responsive to microprocessor control for periodically flushing logged oil in the evaporators of a refrigeration system to return such oil to the compressor, all without disturbing the normal operations of the system.